JET STREAMS, ROSSBY WAVES & POLAR VORTEX

JET STREAMS

A) Jet stream (a type of Geostrophic Wind)

  • The Coriolisforce increases with increase in the wind velocity.
  • The winds in the upper atmosphere, 2-3 km above the surface, are free from frictional effect of the surface and are controlled by the pressure gradient and the Coriolis
  • When isobars are straight, and when there is no friction, the pressure gradient force is balanced by the Coriolis force, and the resultant wind blows parallel to the isobar (deflection of the wind is maximum).
  • This wind is known as the geostrophic wind. Jet Stream is type of geostrophic wind.

 

 

  • As these winds are geostrophic, i.e. flowing at great speed due to low friction and are subjected to greater Coriolis force.
  • They are deflected significantly, giving rise to three distinct cells called Hadley cell, Ferrell Cell and Polar cell.
  • Instead of one big cell there are three small cells producing the same effect.

 

 

  • Hadley Cell and Polar Cell are thermal in origin (convection). Ferrell Cell is dynamic in origin (Coriolis Force and blocking effect of converging winds). These cells are part of general circulation.

 

 

  • Jet Streams are formed due to pressure difference between air masses and Coriolis Force.

 

 

B) Upper tropospheric westerlies

  • Jet streams are produced due to winds flowing from tropics towards poles in the upper troposphere.
  • Jet stream produced between polar and temperate air masses is called as polar jet stream or polar jet and those between temperate and tropical air masses is called as subtropical jet stream.

 

 

  • In polar jet streams wind flows from temperate region towards polar region, and in subtropical jet streams, winds flow from subtropics towards temperate region.
  • In the upper troposphere, the wind flows from less denser air mass towards the poles due to differential heating of earth surface.

 

 

 

 

  • Anything moving from tropics towards poles deflects towards their right in the northern hemisphere and towards their left in the southern hemisphere due to Coriolis effect.
  • Thus, jet streams flow from west to east in both the hemispheres and hence they are called westerlies or upper-level
  • Both the Northern and Southern hemispheres have jet streams, although the jet streams in the north are more forceful due to greater temperature gradients.
  • Polar jet streams flow 7 – 9 km above the ground and Sub-tropical jet streams flows 12 – 14 km above the grounds because the troposphere is thicker at equator (18 km) than at poles (9 km).

C) Jet Streams are very high velocity winds

  • High velocity of jet streams can be attributed to the reduced frictional force in the upper troposphere. The greater the difference in air temperature, the faster the jet stream.
  • Jet stream can reach speeds of up to 400 kmph or greater. The jet streams have an average velocity of 120 kmph in winter and 50 kmph in summer.

D) Meandering of Jet Streams

  • When the temperature contrast is maximum, pressure gradient also becomes high, jet stream velocity will be high thus flowing in near straight path.
  • But when temperature contrast reduces (jet stream is weak), the jet stream starts to follow a meandering path (wavy)
  • Thus, meandering depends on temperature gradient.

 

ROSSBY WAVES

  • The meandering jet streams are called Rossby Waves.
  • It is a natural phenomenon in the atmosphere and oceans due to rotation of earth.
  • In Rossby waves the polar air moves toward the equator while tropical air moves poleward.
  • A meander is called peak or ridge if it is towards poles and trough if it is towards equator.
  • The existence of these waves explains the low-pressure cells (cyclones) and high-pressure cells (anticyclones).

 

 

 

PERMANENT JET STREAMS

  • Polar jet and subtropical jet are permanent jet streams that breeze through the upper troposphere for most part of the year.

 

 

A) Subtropical jet stream (STJ)

  • During winter, the STJ is nearly continuous in both hemispheres.
  • The STJ exists all year in the southern hemisphere. It is intermittent in the northern hemisphere during summer when it migrates north.
  • STJ is closely connected to the Indian and African summer monsoons.

B) Polar jet stream (PJS)

  • The strongest jet streams are the polar jets.But It has more variable position than the sub-tropical jet. The jet is stronger and continuous in winters.
  • The northern Polar jet stream follows the sun, i.e., it slowly migrates northward in summer, and southward towards the equator in winter.
  • The polar jet stream is closely related to the polar front (frontogenesis process in mid-latitudes).
  • It greatly influences climates of regions lying close to 60° latitude.
  • It determines the path and speed and intensity of temperate cyclones.

 

JET STREAM AND WEATHER

A) Influence of Jet Streams on Weather

  • Jet streams help in maintenanceof latitudinal heat balance by mass exchange of air.
  • Sub-tropical jet stream and temporary jet streams together influence Indian Monsoon patterns.
  • Jet streams also exercise an influence on movement of air masses which may cause prolonged drought or flood conditions.

B) Jet Streams and Weather in Temperate Regions

  • PJS play a key role in determining the weather because they usually separate colder air and warmer air.
  • Jet streams generally push air masses around, moving weather systems to new areas and even causing them to stall if they have moved too far away.
  • PJS play a major role in determining the path and intensity of frontal precipitation and frontal cyclones.
  • Weak PJS also results in slipping of polar vortex into temperate regions.
  • The jet stream drives temperate weather through phenomena called troughing, ridging, and jet streaks.
  • Ridges occur where the warm air (at high-pressure) pushes against the cold air.
  • Troughs occur where cold air (at lower pressure) drops into warm air.
  • This condition occurs due to weak jet stream.

 

 

  • Troughs and ridges are analogous to low-pressure (troughs) and high-pressure (ridges).
  • Active weather occurs ahead of a trough and quiet weather beneath a ridge.
  • The ridges and troughs give rise to jet streaks.
  • They form in response to localised but major temperature-gradients.
  • The process of winds exiting a trough or a jet streak, known as divergence, creates a void in the upper atmosphere. Air will rush up from lower altitudes to fill the void.
  • This upward rush of air from the surface creates a low-pressure system.
  • The Coriolis effect creates the cyclonic rotation that is associated with depressions.
  • The winds entering the jet streak are rapidly converging, creating a high-pressure at the upper level in the atmosphere. This leads to divergence (high-pressure) at the surface (anticyclonic condition).
  • The Coriolis effect creates the anticyclonic rotation that is associated with clear weather.

C) Jet Streams and Aviation

  • Jet streams are used by aviators if they have to fly in the direction of the flow of the jet streams and avoid them when flying in opposite direction.
  • Jet streams can cause a flight turbulenceas their movement is unpredictable, even when the weather looks calm and clear.
  • During volcanic eruptions plumes of volcanic ash tend to get sucked into the same jet stream that aeroplanes use for travel.

 

POLAR VORTEX

  • Polar vortex is a polar cyclone. That can reach up to 2,000 km wide.
  • Polar cyclones are not seasonal. They can occur at any time of the year.
  • Polar cyclones can also form quickly (sometimes less than 24 hours), and their direction or movement cannot be predicted.
  • They can last from a day up to several weeks.
  • Most frequently, polar cyclones develop above northern Russia and Siberia.
  • A polar vortex is a large parcel of extremely cold air, which sits over the polar region during the winter season.
  • Polar Vortex is a
  • Cold;
  • Circumpolar;
  • Upper tropospheric low-pressure: occasionally extending till the lower levels of stratosphere
  • Large cyclonic parcel of air (about 1000 km across) (counter-clockwise in the Northern Hemisphere)
  • Polar vortex is closely associated with jet streams (Rossby waves).
  • It is formed mainly in winter and gets weaker in summer.
  • It surrounds polar highs and lie within the polar front (boundary separating the temperate and polar air masses).

 

 

 

 

A) How does it collapse/ breaks?

  • The Polar jet traverses somewhere over 65º N and S latitudes. When the temperature contrast between polar and temperate regions is maximum, the jet is very strong, and the meandering is negligible.
  • But when the temperature contrast is low, the jet starts to meander
  • Meandering jet creates alternating low and high-pressure
  • High-pressure cells are created below the ridges and the low-pressure cells below the troughs
  • With severe meandering, the high-pressure cells push over to north and displace the polar cyclone from its normal position e. the cyclone moves away from the pole and slips into the temperate regions.
  • With the strengthening of the jet, the high-pressure cells become weak and retreat to their normal latitudinal positions.
  • With the retreat of the high-pressure cells, the polar cyclone moves back to its normal position.

 

 

B) Ozone depletion and polar vortex

  • There is a steady decline of 4% of the total volume of ozone in Earth’s stratosphere butgreaterdeclineis seen in stratospheric ozone around Earth’s polar regions.
  • Depletion of ozone is due to increase in halocarbons in the atmosphere.

 

 

  • Halocarbon are the compound in which the hydrogen of a hydrocarbon is replaced by halogens like chlorine, bromine, iodine. Halogen atoms like chlorine destroy ozone
  • Photodissociation (sunlight) of ozone-depleting substances (ODS) like halocarbon refrigerants, solvents, propellants, and foaming agents (CFCs, HCFCs, CCL4 ,trichloroethane, freons, halons) creates free chlorine atoms that speed up the process of ozone depletion.

 

 

 

 

 

C) Polar Stratospheric Clouds (PSCs)

 

 

  •  They are nacreous clouds (rare/in frigid region) extending from 12-22 km above the surface.
  • They are mostly visible within two hours after sunset or before dawn.
  • They are bright even after sunset and before dawn because at those heights there is still sunlight.
  • PSCs contains water droplets, nitric acid and/or sulfuric acid.
  • They are formed mainly during the event of polar vortex in winter and are more intense at south pole.
  • The Cl-catalysed ozone depletion is enhanced in the presence of polar stratospheric clouds.
  • PSCs convert reservoir compounds into reactive free radicals (Cl and ClO) thereby significantly increasing the reactive free halogen radicals. These free radicals accelerate depletion of ozone.
  • Thus, polar vortex, in the form of PSCs, accelerate ozone depletion.
  • Ozonosphere lies at an altitude between 20 km and 55 km from the earth’s surface and spans the stratosphere and lower mesosphere. But the highest concentration occurs between 20 km and 30 km.
  • To destroy ozone, ozone-depleting substances (ODS) like CFCs, HCFCs, etc. needs to be carried up to the lower levels of stratosphere. Polar vortex can reach to this altitude.

 

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